Streaming adaption based on clean random access (cra) pictures

ABSTRACT

Systems, methods, and devices for processing video data are disclosed. Some examples systems, methods, and devices receive an external indication at a video decoder. The example systems, methods, and devices treat a clean random access (CRA) picture as a broken link access (BLA) picture based on the external indication.

This application claims the benefit of:

U.S. Provisional Application No. 61/665,667, filed Jun. 28, 2012, theentire content of which is incorporated herein by reference.

TECHNICAL FIELD

This disclosure generally relates to processing video data and, moreparticularly, to techniques for supporting random access in compressedvideo streams.

BACKGROUND

Digital video capabilities can be incorporated into a wide range ofdevices, including digital televisions, digital direct broadcastsystems, wireless broadcast systems, personal digital assistants (PDAs),laptop or desktop computers, tablet computers, e-book readers, digitalcameras, digital recording devices, digital media players, video gamingdevices, video game consoles, cellular or satellite radio telephones,so-called “smart phones,” video teleconferencing devices, videostreaming devices, transcoders, routers or other network devices, andthe like. Digital video devices implement video compression techniques,such as those described in the standards defined by MPEG-2, MPEG-4,ITU-T H.263, ITU-T H.264/MPEG-4, Part 10, Advanced Video Coding (AVC),the High Efficiency Video Coding (HEVC) standard presently underdevelopment, proprietary standards, open video compression formats suchas VP8, and extensions of such standards, techniques, or formats. Thevideo devices may transmit, receive, encode, decode, and/or storedigital video information more efficiently by implementing such videocompression techniques.

Video compression techniques perform spatial (intra-picture) predictionand/or temporal (inter-picture) prediction to reduce or removeredundancy inherent in video sequences. For block-based video coding, avideo slice (i.e., a video picture or a portion of a video picture) maybe partitioned into video blocks, which may also be referred to astreeblocks, coding units (CUs) and/or coding nodes. Video blocks in anintra-coded (I) slice of a picture are encoded using spatial predictionwith respect to reference samples in neighboring blocks in the samepicture. Video blocks in an inter-coded (P or B) slice of a picture mayuse spatial prediction with respect to reference samples in neighboringblocks in the same picture or temporal prediction with respect toreference samples in other reference pictures. Pictures may be referredto as frames, and reference pictures may be referred to as referenceframes.

Spatial or temporal prediction results in a predictive block for a blockto be coded. Residual data represents pixel differences between theoriginal block to be coded and the predictive block. An inter-codedblock is encoded according to a motion vector that points to a block ofreference samples forming the predictive block, and the residual dataindicating the difference between the coded block and the predictiveblock. An intra-coded block is encoded according to an intra-coding modeand the residual data. For further compression, the residual data may betransformed from the pixel domain to a transform domain, resulting inresidual transform coefficients, which then may be quantized. Thequantized transform coefficients, initially arranged in atwo-dimensional array, may be scanned in order to produce aone-dimensional vector of transform coefficients, and entropy coding maybe applied to achieve even more compression.

SUMMARY

In one example, the techniques of this disclosure relate to treating aclean random access (CRA) picture as a broken link access (BLA) picturebased on an external indication. For example, a video decoder or otherdevice may receive an external indication. The video decoder may thentreat a CRA picture as a BLA picture based on the external indication.In some examples, a flag is defined for a CRA picture and the externalindication indicates whether the flag should be set in the videodecoder. Accordingly, the video decoder may set the flag based on theexternal indication. The decoder or some internal functionality, such asan external indication processing unit or a prediction module may thencheck the flag. In an example, the prediction module may treat a CRApicture as a BLA picture based on the external indication. For example,a decoder may treat the CRA picture as a BLA picture based on the flag.

In one example, the disclosure describes a method of processing videodata that includes receiving an external indication at a video decoderand treating a clean random access (CRA) picture as a broken link access(BLA) picture based on the external indication.

In another example, the disclosure describes a video decoder forprocessing video data, including a processor configured to receive anexternal indication at a video decoder and treat a clean random access(CRA) picture as a broken link access (BLA) picture based on theexternal indication.

In another example, the disclosure describes a video decoder forprocessing video data that includes means for receiving an externalindication at a video decoder and means for treating a clean randomaccess (CRA) picture as a broken link access (BLA) picture based on theexternal indication.

In another example, the disclosure describes a computer-readable storagemedium. The computer-readable storage medium having stored thereoninstructions that upon execution cause one or more processors of adevice to receive an external indication at a video decoder and treat aclean random access (CRA) picture as a broken link access (BLA) picturebased on the external indication.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system that may utilize the techniques described in thisdisclosure.

FIG. 2 is a block diagram illustrating an example video encoder that mayimplement the techniques described in this disclosure.

FIG. 3 is a block diagram illustrating an example video decoder that mayimplement the techniques described in this disclosure.

FIG. 4 is a block diagram illustrating an example set of devices thatform part of a network.

FIG. 5 is a flowchart illustrating an example method in accordance withone or more examples described in this disclosure.

FIG. 6 is a flowchart illustrating an example method in accordance withone or more examples described in this disclosure.

FIG. 7 is a flowchart illustrating an example method in accordance withone or more examples described in this disclosure.

FIG. 8 is a flowchart illustrating an exemplary operation of a firstdevice sending an external indication and responsive actions of a seconddevice receiving the external indication.

DETAILED DESCRIPTION

This disclosure describes techniques for streaming adaption based onclean random access (CRA) pictures. Various improved video codingdesigns are described, which may be related to streaming adaptationbased on CRA pictures, output of pictures before random access point(RAP) pictures, and signaling of picture timing information.

A brief background of some video coding standards is first described.Video coding standards include ITU-T H.261, ISO/IEC MPEG-1 Visual, ITU-TH.262 or ISO/IEC MPEG-2 Visual, ITU-T H.263, ISO/IEC MPEG-4 Visual andITU-T H.264 (also known as ISO/IEC MPEG-4 AVC), including its ScalableVideo Coding (SVC) and Multiview Video Coding (MVC) extensions.

In addition, there is a new video coding standard, namelyHigh-Efficiency Video Coding (HEVC), being developed by the JointCollaboration Team on Video Coding (JCT-VC) of ITU-T Video CodingExperts Group (VCEG) and ISO/IEC Motion Picture Experts Group (MPEG). AWorking Draft (WD) of HEVC, and referred to as HEVC WD7 hereinafter, isavailable fromhttp://phenix.int-evry.fr/jct/doc_end_user/documents/9_Geneva/wg11/JCTVC-11003-v5.zip,the entire content of which is incorporated herein by reference.

A more recent Working Draft (WD) of HEVC, and referred to as HEVC WD9hereinafter, is available fromhttp://phenix.int-evry.fr/jct/doc_end_user/documents/9_Geneva/wg11/JCTVC-11003-v10.zip,the entire content of which is incorporated herein by reference.

In one example, the techniques of this disclosure relate to treating aclean random access (CRA) picture as a broken link access (BLA) picturebased on an external indication. For example, a video decoder or otherdevice may receive an external indication. The video decoder may thentreat a CRA picture as a BLA picture based on the external indication.In some examples, a flag is defined for a CRA picture and the externalindication indicates whether the flag should be set in the videodecoder. Accordingly, the video decoder may set the flag based on theexternal indication. The decoder or some internal functionality, such asan external indication processing unit or a prediction module may thencheck the flag. In an example, the prediction module may treat a CRApicture as a BLA picture based on the external indication. For example,a decoder may treat the CRA picture as a BLA picture based on the flag.

In another example, a flag is defined for a CRA picture and a decoder orother device may receive an external indication that the flag should beset. The decoder or other device may then set the flag based on theexternal indication. The decoder may then check the flag. When the flagis set, the decoder may treat the CRA picture as a BLA picture.

Random access refers to the decoding of a video bitstream starting froma coded picture that is not the first coded picture in the bitstream.Random access to a bitstream is needed in many video applications, suchas broadcasting and streaming, e.g., for users to switch betweendifferent channels, to jump to specific parts of the video, or to switchto a different bitstream for stream adaptation (e.g., of the bit rate,frame rate, spatial resolution, and so on). This feature may be enabledby inserting random access pictures or random access points, many timesin regular intervals, into the video bitstream.

Bitstream splicing refers to the concatenation of two or more bitstreamsor parts thereof. For example, a first bitstream may be appended to asecond bitstream, possibly with some modifications to either one or bothof the bitstreams, to generate a spliced bitstream. The first codedpicture in the second bitstream is also referred to as the splicingpoint. Therefore, pictures following the splicing point in the splicedbitstream originate from the second bitstream while pictures precedingthe splicing point in the spliced bitstream originate from the firstbitstream.

Bitstream splicers may perform splicing of bitstreams. Bitstreamsplicers are often less complicated, less sophisticated and/or lessintelligent than encoders. For example, they may not be equipped withentropy decoding and encoding capabilities. Bitstream splicers may beincorporated into any of the devices described herein, including codingdevices or network devices.

Bitstream switching may be used in adaptive streaming environments. Abitstream switching operation at a certain picture in the switch-tobitstream is effectively a bitstream splicing operation wherein thesplicing point is the bitstream switching point, i.e., the first picturefrom the switch-to bitstream.

Instantaneous decoding refresh (IDR) pictures as specified in AVC orHEVC can be used for random access. However, because pictures followingan IDR picture in a decoding order cannot use pictures decoded prior tothe IDR picture as a reference, bitstreams relying on IDR pictures forrandom access can have significantly lower coding efficiency.

To improve coding efficiency, the concept of clean random access (CRA)pictures was introduced in HEVC to allow pictures that follow a CRApicture in decoding order but precede it in output order to use picturesdecoded before the CRA picture as a reference. Pictures that follow aCRA picture in decoding order but precede the CRA picture in outputorder are referred to as leading pictures associated with the CRApicture (or leading pictures of the CRA picture). The leading picturesof a CRA picture are correctly decodable if the decoding starts from anIDR or CRA picture before the current CRA picture. However, the leadingpictures of a CRA picture may be non-correctly-decodable when randomaccess from the CRA picture occurs. Hence, decoders typically discardthe leading pictures during random access decoding. To prevent errorpropagation from reference pictures that may not be available dependingon where the decoding starts, all pictures that follow a CRA pictureboth in decoding order and output order shall not use any picture thatprecedes the CRA picture either in decoding order or output order (whichincludes the leading pictures) as reference.

The concept of a broken link access (BLA) picture was further introducedin HEVC after the introduction of CRA pictures and based on the conceptof CRA pictures. A BLA picture typically originates from bitstreamsplicing at the position of a CRA picture, and in the spliced bitstream,the splicing point CRA picture may be changed to a BLA picture. IDRpicture, CRA picture and BLA picture are collectively referred to asrandom access point (RAP) pictures.

One difference between BLA pictures and CRA pictures is as follows. Fora CRA picture, the associated leading pictures are correctly decodableif the decoding starts from a RAP picture before the CRA picture indecoding order. The CRA picture may be non-correctly-decodable whenrandom access from the CRA picture occurs. For example, when thedecoding starts from the CRA picture, or in other words, when the CRApicture is the first picture in the bitstream. For a BLA picture, theassociated leading pictures may be non-correctly-decodable in all cases,even when the decoding starts from a RAP picture before the BLA picturein decoding order.

For a particular CRA or BLA picture, some of the associated leadingpictures are correctly decodable even when the CRA or BLA picture is thefirst picture in the bitstream. These leading pictures are referred toas decodable leading pictures (DLPs), and other leading pictures arereferred to as non-decodable leading pictures (NLPs) or random accessdecodable leading (RADL) pictures. NLPs are also referred to as taggedfor discard (TFD) pictures or as random access skipping leading (RASL)pictures.

In some cases, the following problems may be associated with someexisting methods (1) in streaming adaptation based on CRA pictures,changing of a CRA picture to a BLA picture typically needs to beperformed by a media server or an intermediate network element, e.g. amedia-aware network element (MANE) or even a media-unaware networkelement such as an HTTP cache or web proxy, MANE, which is typicallypreferable to be less complicated, less sophisticated and/or lessintelligent and may not be able to change the bitstream at all, (2)output of pictures before an IDR or BLA picture in decoding order can besomehow controlled by using the no_output_of_prior_pics_flag. Whenno_output_of_prior_pics_flag is set to “1” or inferred to be equal to 1,decoded pictures earlier in decoding order than the IDR or BLA pictureare all discarded after decoding of the IDR or BLA picture withoutoutput/display. However, sometimes displaying more of those pictures mayprovide better user experience. Currently there is not a way to enableoutput/display more picture in such situations, (3) DLP pictures areallowed to be output. Since their output order or output times areearlier than the associated RAP picture, the earliest presentation timewhen random accessing from the RAP picture cannot be known by simplychecking the access unit containing the RAP picture. However, whenrandom accessing from a RAP picture, the system should try to figure outthe earliest playback start to see whether that RAP picture fits therandom access request from the user.

A number of techniques are set forth in this disclosure that maygenerally address or improve upon one or more of the above identifiedproblems. A number of different ways of handling such a message,received or inferred, are possible. Several examples are discussedbelow; these include (1) handling a CRA picture as a BLA picture, (2)changing of a CRA picture to a BLA picture, and (3) handling a CRApicture as a CRA picture that starts a bitstream.

In an example, a decoder may handle a CRA picture as a BLA picture. Adecoder may be configured such that a CRA picture is handled as a BLApicture when it is indicated so by an external means. Such an externalindication can be a message as described above (that certain CRA pictureshould be handled as a BLA picture) that is passed to the decoder, by afunction of the decoder side, through inference or reception from aserver or an intermediate network element.

More specifically, the decoding process may be changed to be as follows.A separate variable that may be associated with each CRA picture may beused. For example, the variable HandleCraAsBlaFlag is associated witheach CRA picture. In other words, each CRA picture may have aHandleCraAsBlaFlag variable (also referred to as a flag) associated withit. The value of HandleCraAsBlaFlag for some CRA pictures may bespecified by external means. When the value of HandleCraAsBlaFlag for aparticular CRA picture is not specified by external means, it may be setto “0” (e.g., HandleCraAsBlaFlag of a CRA picture by default is “0,”with the value “0” indicating that a CRA picture is not treated as a BLApicture). In such an example, a value of “1” may indicate that a CRApicture is treated as a BLA picture. In other examples, the opposite maybe true, a value of “1” may indicate that a CRA picture is not treatedas a BLA picture and a value of “0” may indicate that a CRA picture istreated as a BLA picture.

The following example assumes the case when HandleCraAsBlaFlag defaultsto a value of “0” indicating that a CRA picture is not treated as a BLApicture and a value of “1” indicating that a CRA picture is treated as aBLA picture. When decoding (including parsing) each coded slice NALunit, if HandleCraAsBlaFlag is equal to “1,” e.g., handle a CRA pictureas a BLA picture, and nal_unit_type indicates a CRA picture (e.g. thevalue is equal to “4” or “5” according to HEVC WD7), the followingapplies, (1) the value of nal_unit_type is changed to indicate a BLApicture (e.g. the value is increased by 2 according to HEVC WD7), (2)the value of no_output_of_prior_pics_flag is set to 1, (3) if theprevious picture in decoding order is a RAP picture and the rap_pic_idof the current slice is equal to the rap_pic_id of the previous picture,the following applies. First, if the next picture in decoding order isnot a RAP picture, the value of rap_pic_id of the current slice ischanged to be different than the rap_pic_id of the previous picture indecoding order, but still in the allowed value range of the syntaxelement. Second, otherwise (the next picture in decoding order is a RAPpicture), the value of rap_pic_id of the current picture is changed tobe a value that is different than the rap_pic_id of both the previouspicture and the next picture in decoding order, but still in the allowedvalue range of the syntax element.

Alternatively, when changing of a CRA picture to a BLA picture, adecoder may perform the following, if the picture timing SEI messagesare present and the DPB output times for all pictures in the DPB aresmaller than the DPB output time of the current picture, the value ofno_output_of_prior_pics_flag is set to 1; otherwise if the value ofno_output_of_prior_pics_flag is set to “0”.

In some examples, HandleCraAsBlaFlag may be a first flag and theno_output_of_prior_pictures_flag may be a second flag. In some examples,the no_output_of_prior_pictures_flag may be a context variable.

With the above changes to HEVC WD7, it may be possible to further removethe special decoding processes for a CRA picture that is the firstpicture in the bitstream and the associated TFD pictures. In this case,when a bitstream starts with a CRA picture, the first CRA picture in thebitstream should be handled as a BLA picture, by setting the value ofHandleCraAsBlaFlag to “1” for the bitstream-starting CRA picture,regardless of whether the value is specified by the external means, ifany, and applying the above changed decoding process.

Alternatively, when decoding (including parsing) each coded slice NALunit, if the current picture is the first picture in the bitstream andnal_unit_type indicates a CRA picture (e.g. the value is equal to “4” or“5” per HEVC WD7), the following may apply, the value of nal_unit_typeis changed to indicate a BLA picture (e.g. the value is increased by 2per HEVC WD5). In this example, there is no need to change the values ofno_output_of_prior_pics_flag and rap_pic_id. Alternatively, the value ofHandleCraAsBlaFlag may be indicated by a syntax element in thebitstream, e.g. a new syntax element that may be included in the sliceheader or a new SEI message.

One example relates to a streaming adaptation based on CRA pictures. Insuch an example, instead of relying on a server or an intermediatenetwork element to change a BLA picture to a CRA picture, a server or anintermediate network element may generate a message to be sent to thedecoder side (i.e. the client). The message may notify a decoder, forexample, that a bitstream switching operation has occurred at certainCRA picture and that CRA picture should be handled as a BLA picture. Inthe context of dynamic adaptive streaming over HTTP (DASH), the decoderside may also infer such a message by itself through the change of theuniform resource locator (URL) it used for requesting stream data andthe reception of the media data associated with the changed URL.

In another example, a CRA picture may be changed such that if picturetiming SEI messages are present and the DPB output times for allpictures in the DPB are smaller than the DPB output time of the currentpicture. The value of no_output_of_prior_pics_flag may be set to 1.Otherwise the value of no_output_of_prior_pics_flag may be set to “0”.

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system 10 that may utilize the techniques described in thisdisclosure. As shown in FIG. 1, system 10 includes a source device 12that generates encoded video data to be decoded at a later time by adestination device 14. The techniques described herein generally relateto treating a CRA picture as a BLA picture based on an externalindication. Accordingly, these techniques may generally apply to thedestination device 14, which may generally receive the externalindication and in response to such an external indication, may treat aCRA picture received at the destination device as a BLA picture whenprocessed within the destination device. In some examples however,source device 12 or another network device, such as a MANE may providean external indication to destination device 14 that causes destinationdevice 14 to treat a CRA picture received at the destination device as aBLA picture.

Source device 12 and destination device 14 may comprise any of a widerange of devices, including desktop computers, notebook (i.e., laptop)computers, tablet computers, set-top boxes, telephone handsets such asso-called “smart” phones, so-called “smart” pads, televisions, cameras,display devices, digital media players, video gaming consoles, videostreaming device, or the like. In some cases, source device 12 anddestination device 14 may be equipped for wireless communication.

Destination device 14 may receive the encoded video data to be decodedvia a link 16. Link 16 may comprise any type of medium or device capableof moving the encoded video data from source device 12 to destinationdevice 14. In one example, link 16 may comprise a communication mediumto enable source device 12 to transmit encoded video data directly todestination device 14 in real-time. A modulator may modulate the encodedvideo data according to a communication standard, such as a wirelesscommunication protocol, and transmitted to destination device 14. Thecommunication medium may comprise any wireless or wired communicationmedium, such as a radio frequency (RF) spectrum or one or more physicaltransmission lines. The communication medium may form part of apacket-based network, such as a local area network, a wide-area network,or a global network such as the Internet. The communication medium mayinclude routers, switches, base stations, or any other equipment thatmay be useful to facilitate communication from source device 12 todestination device 14.

Alternatively, encoded data may be output from output interface 22 to astorage device 32. Similarly, input interface may access encoded datafrom storage device 32. Storage device 36 may include any of a varietyof distributed or locally accessed data storage media such as a harddrive, Blu-ray discs, DVDs, CD-ROMs, flash memory, volatile ornon-volatile memory, or any other suitable digital storage media forstoring encoded video data. In a further example, storage device 36 maycorrespond to a file server or another intermediate storage device thatmay hold the encoded video generated by source device 12. Destinationdevice 14 may access stored video data from storage device 36 viastreaming or download. The file server may be any type of server capableof storing encoded video data and transmitting that encoded video datato the destination device 14. Example file servers include a web server(e.g., for a website), an FTP server, network attached storage (NAS)devices, or a local disk drive. Destination device 14 may access theencoded video data through any standard data connection, including anInternet connection. This may include a wireless channel (e.g., a Wi-Ficonnection), a wired connection (e.g., DSL, cable modem, etc.), or acombination of both that is suitable for accessing encoded video datastored on a file server. The transmission of encoded video data fromstorage device 36 may be a streaming transmission, a downloadtransmission, or a combination of both.

The techniques of this disclosure are not necessarily limited towireless applications or settings. The techniques may be applied tovideo coding in support of any of a variety of multimedia applications,such as over-the-air television broadcasts, cable televisiontransmissions, satellite television transmissions, streaming videotransmissions, e.g., via the Internet, encoding of digital video forstorage on a data storage medium, decoding of digital video stored on adata storage medium, or other applications. In some examples, system 10may be configured to support one-way or two-way video transmission tosupport applications such as video streaming, video playback, videobroadcasting, and/or video telephony.

In the example of FIG. 1, source device 12 includes a video source 18,video encoder 20 and an output interface 22. In some cases, outputinterface 22 may include a modulator/demodulator (modem) and/or atransmitter. In source device 12, video source 18 may include a sourcesuch as a video capture device. For example, a video camera, a videoarchive containing previously captured video, a video feed interface toreceive video from a video content provider, and/or a computer graphicssystem for generating computer graphics data as the source video, or acombination of such sources. As one example, if video source 18 is avideo camera, source device 12 and destination device 14 may formso-called camera phones or video phones. However, the techniquesdescribed in this disclosure may be applicable to video coding ingeneral, and may be applied to wireless and/or wired applications.

Video encoder 20 may encode the captured, pre-captured, orcomputer-generated video. The encoded video data may be transmitteddirectly to destination device 14 via output interface 22 of sourcedevice 12. Alternatively, the encoded video data may be stored ontostorage device 36 for later access by destination device 14 or otherdevices, for decoding and/or playback. In other examples both of thesemay be performed.

Destination device 14 includes an input interface 28, a video decoder30, and a display device 32. In some cases, input interface 28 mayinclude a receiver and/or a modem. Input interface 28 of destinationdevice 14 receives the encoded video data over link 16. The encodedvideo data communicated over link 16, or provided on storage device 36,may include a variety of syntax elements generated by video encoder 20for use by a video decoder, such as video decoder 30, in decoding thevideo data. Such syntax elements may be included with the encoded videodata transmitted on a communication medium, stored on a storage medium,or stored a file server.

In one example, video decoder 30 or other device may receive an externalindication. Video decoder 30 may then treat a clean random access (CRA)picture as a broken link access (BLA) picture based on the externalindication. In some examples, the external indication indicates whethera flag should be set in the video decoder. Accordingly, the videodecoder 30 may set the flag based on the external indication. The videodecoder 30 may or some internal functionality, such as an externalindication processing unit 72 or a prediction module 81 may then checkthe flag. In an example, the prediction module 81 may treat a CRApicture as a BLA picture based on the external indication that indicatesthat the CRA picture should be treated as a BLA pictures based on theflag.

In another example, video decoder 30 or another device may receive anexternal indication that a flag should be set. Video decoder 30 oranother device may then set the flag based on the external indication.Decoder 30 may then check the flag. When the flag is set video decoder30 treats the CRA picture as a BLA picture.

Display device 32 may be integrated with, or external to, destinationdevice 14. In some examples, destination device 14 may include anintegrated display device and also be configured to interface with anexternal display device. In other examples, destination device 14 may bea display device. In general, display device 32 displays the decodedvideo data to a user, and may comprise any of a variety of displaydevices such as a liquid crystal display (LCD), a plasma display, anorganic light emitting diode (OLED) display, or another type of displaydevice.

Video encoder 20 and video decoder 30 may operate according to a videocompression standard, such as the High Efficiency Video Coding (HEVC)standard presently under development, and may conform to the HEVC TestModel (HM). A recent draft of HEVC is available, as of Jun. 27, 2012,fromhttp://wg11.sc29.org/jct/doc_end_user/current_document.php?id=5885/JCTVC-11003-v5,the entire content of which is incorporated herein by reference.Alternatively, video encoder 20 and video decoder 30 may operateaccording to other proprietary or industry standards, such as the ITU-TH.264 standard, alternatively referred to as MPEG-4, Part 10, AdvancedVideo Coding (AVC), or extensions of such standards. The techniques ofthis disclosure, however, are not limited to any particular codingstandard. Other examples of video compression standards include MPEG-2and ITU-T H.263, as well as open formats such as VP8.

Although not shown in FIG. 1, in some aspects, video encoder 20 andvideo decoder 30 may each be integrated with an audio encoder anddecoder, and may include appropriate MUX-DEMUX units, or other hardwareand software, to handle encoding of both audio and video in a commondata stream or separate data streams. If applicable, in some examples,MUX-DEMUX units may conform to the ITU H.223 multiplexer protocol, orother protocols such as the user datagram protocol (UDP).

Video encoder 20 and video decoder 30 each may be implemented as any ofa variety of suitable encoder circuitry, such as one or moremicroprocessors, digital signal processors (DSPs), application specificintegrated circuits (ASICs), field programmable gate arrays (FPGAs),discrete logic, software, hardware, firmware or any combinationsthereof. When the techniques are implemented partially in software, adevice may store instructions for the software in a suitable,non-transitory computer-readable medium and execute the instructions inhardware using one or more processors to perform the techniques of thisdisclosure. Each of video encoder 20 and video decoder 30 may beincluded in one or more encoders or decoders, either of which may beintegrated as part of a combined encoder/decoder (CODEC) in a respectivedevice.

The JCT-VC is working on development of the HEVC standard. The HEVCstandardization efforts are based on an evolving model of a video codingdevice referred to as the HEVC Test Model (HM). The HM presumes severaladditional capabilities of video coding devices relative to existingdevices according to, e.g., ITU-T H.264/AVC. For example, whereas H.264provides nine intra-prediction encoding modes, the HM may provide asmany as thirty-three intra-prediction encoding modes.

In general, the working model of the HM describes that a video frame orpicture may be divided into a sequence of coding tree blocks ortreeblocks or largest coding units (LCU) that include both luma andchroma samples. A treeblock may have a similar purpose as a macroblockof the H.264 standard. A slice includes a number of consecutivetreeblocks in coding order. A video frame or picture may be partitionedinto one or more slices. Each treeblock may be split into coding units(CUs) according to a quadtree. For example, a treeblock, as a root nodeof the quadtree, may be split into four child nodes, and each child nodemay in turn be a parent node and be split into another four child nodes.A final, unsplit child node, as a leaf node of the quadtree, comprises acoding node, i.e., a coded video block. Syntax data associated with acoded bitstream may define a maximum number of times a treeblock may besplit, and may also define a minimum size of the coding nodes.

A CU includes a coding node and prediction units (PUs) and transformunits (TUs) associated with the coding node. A size of the CUcorresponds to a size of the coding node and must be square in shape.The size of the CU may range from 8×8 pixels up to the size of thetreeblock with a maximum of 64×64 pixels or greater. Each CU may containone or more PUs and one or more TUs. Syntax data associated with a CUmay describe, for example, partitioning of the CU into one or more PUs.Partitioning modes may differ between whether the CU is skip or directmode encoded, intra-prediction mode encoded, or inter-prediction modeencoded. PUs may be partitioned to be non-square in shape. Syntax dataassociated with a CU may also describe, for example, partitioning of theCU into one or more TUs according to a quadtree. A TU may be square ornon-square in shape.

The HEVC standard allows for transformations according to TUs, which maybe different for different CUs. The TUs are typically sized based on thesize of PUs within a given CU defined for a partitioned LCU, althoughthis may not always be the case. The TUs are typically the same size orsmaller than the PUs. In some examples, residual samples correspondingto a CU may be subdivided into smaller units using a quadtree structureknown as “residual quad tree” (RQT). The leaf nodes of the RQT may bereferred to as transform units (TUs). Pixel difference values associatedwith the TUs may be transformed to produce transform coefficients, whichmay be quantized.

In general, a PU includes data related to the prediction process. Forexample, when the PU is intra-mode encoded, the PU may include datadescribing an intra-prediction mode for the PU. As another example, whenthe PU is inter-mode encoded, the PU may include data defining a motionvector for the PU. The data defining the motion vector for a PU maydescribe, for example, a horizontal component of the motion vector, avertical component of the motion vector, a resolution for the motionvector (e.g., one-quarter pixel precision or one-eighth pixelprecision), a reference picture to which the motion vector points,and/or a reference picture list (e.g., List 0, List 1, or List C) forthe motion vector.

In general, a TU is used for the transform and quantization processes. Agiven CU having one or more PUs may also include one or more transformunits (TUs). Following prediction, video encoder 20 may calculateresidual values corresponding to the PU. The residual values comprisepixel difference values that may be transformed into transformcoefficients, quantized, and scanned using the TUs to produce serializedtransform coefficients for entropy coding. This disclosure typicallyuses the term “video block” to refer to a coding node of a CU. In somespecific cases, this disclosure may also use the term “video block” torefer to a treeblock, i.e., LCU, or a CU, which includes a coding nodeand PUs and TUs.

A video sequence typically includes a series of video frames orpictures. A group of pictures (GOP) generally comprises a series of oneor more of the video pictures. A GOP may include syntax data in a headerof the GOP, a header of one or more of the pictures, or elsewhere, thatdescribes a number of pictures included in the GOP. Each slice of apicture may include slice syntax data that describes an encoding modefor the respective slice. Video encoder 20 typically operates on videoblocks within individual video slices in order to encode the video data.A video block may correspond to a coding node within a CU. The videoblocks may have fixed or varying sizes, and may differ in size accordingto a specified coding standard.

As an example, the HM supports prediction in various PU sizes. Assumingthat the size of a particular CU is 2N×2N, the HM supportsintra-prediction in PU sizes of 2N×2N or N×N, and inter-prediction insymmetric PU sizes of 2N×2N, 2N×N, N×2N, or N×N. The HM also supportsasymmetric partitioning for inter-prediction in PU sizes of 2N×nU,2N×nD, nL×2N, and nR×2N. In asymmetric partitioning, one direction of aCU is not partitioned, while the other direction is partitioned into 25%and 75%. The portion of the CU corresponding to the 25% partition isindicated by an “n” followed by an indication of “Up,” “Down,” “Left,”or “Right.” Thus, for example, “2N×nU” refers to a 2N×2N CU that ispartitioned horizontally with a 2N×0.5N PU on top and a 2N×1.5N PU onbottom.

In this disclosure, “N×N” and “N by N” may be used interchangeably torefer to the pixel dimensions of a video block in terms of vertical andhorizontal dimensions, e.g., 16×16 pixels or 16 by 16 pixels. Ingeneral, a 16×16 block will have 16 pixels in a vertical direction(y=16) and 16 pixels in a horizontal direction (x=16). Likewise, an N×Nblock generally has N pixels in a vertical direction and N pixels in ahorizontal direction, where N represents a nonnegative integer value.The pixels in a block may be arranged in rows and columns. Moreover,blocks need not necessarily have the same number of pixels in thehorizontal direction as in the vertical direction. For example, blocksmay comprise N×M pixels, where M is not necessarily equal to N.

Following intra-predictive or inter-predictive coding using the PUs of aCU, video encoder 20 may calculate residual data for the TUs of the CU.The PUs may comprise pixel data in the spatial domain (also referred toas the pixel domain) and the TUs may comprise coefficients in thetransform domain following application of a transform, e.g., a discretecosine transform (DCT), an integer transform, a wavelet transform, or aconceptually similar transform to residual video data. The residual datamay correspond to pixel differences between pixels of the unencodedpicture and prediction values corresponding to the PUs. Video encoder 20may form the TUs including the residual data for the CU, and thentransform the TUs to produce transform coefficients for the CU.

Following any transforms to produce transform coefficients, videoencoder 20 may perform quantization of the transform coefficients.Quantization generally refers to a process in which transformcoefficients are quantized to possibly reduce the amount of data used torepresent the coefficients, providing further compression. Thequantization process may reduce the bit depth associated with some orall of the coefficients. For example, an n-bit value may be rounded downto an m-bit value during quantization, where n is greater than m.

In some examples, video encoder 20 may utilize a predefined scan orderto scan the quantized transform coefficients to produce a serializedvector that can be entropy encoded. In other examples, video encoder 20may perform an adaptive scan. After scanning the quantized transformcoefficients to form a one-dimensional vector, video encoder 20 mayentropy encode the one-dimensional vector, e.g., according to contextadaptive variable length coding (CAVLC), context adaptive binaryarithmetic coding (CABAC), syntax-based context-adaptive binaryarithmetic coding (SBAC), Probability Interval Partitioning Entropy(PIPE) coding or another entropy encoding methodology. Video encoder 20may also entropy encode syntax elements associated with the encodedvideo data for use by video decoder 30 in decoding the video data.

To perform CABAC, video encoder 20 may assign a context within a contextmodel to a symbol to be transmitted. The context may relate to, forexample, whether neighboring values of the symbol are non-zero or not.To perform CAVLC, video encoder 20 may select a variable length code fora symbol to be transmitted. Codewords in VLC may be constructed suchthat relatively shorter codes correspond to more probable symbols, whilelonger codes correspond to less probable symbols. In this way, the useof VLC may achieve a bit savings over, for example, using equal-lengthcodewords for each symbol to be transmitted. The probabilitydetermination may be based on a context assigned to the symbol.

In accordance with this disclosure, source device 12 (or possiblyanother intermediate device not shown in FIG. 1) may provide an externalindication 34 to destination device 14 that causes destination device 14to treat a CRA picture received at the destination device as a BLApicture. For example, source device 12 (or possibly another intermediatedevice not shown in FIG. 1) may determine that a change has been made bya user, such as requesting video of a different resolution or quality,or a broken link may occur. When a change in resolution or a broken linkoccurs, for example, this may mean that a CRA picture should be treatedas a BLA picture because any prior picture information stored on thereceiving device may not be valid for decoding the incoming bitstream.

FIG. 2 is a block diagram illustrating an example video encoder 20 thatmay implement the techniques described in this disclosure. As discussedabove, the techniques described herein generally relate to treating aCRA picture as a BLA picture based on an external indication received atthe destination device 14. In some examples however, source device 12 oranother network device, such as a MANE may provide an externalindication to destination device 14 that causes destination device 14 totreat a CRA picture received at the destination device as a BLA picture.

Video encoder 20 may perform intra- and inter-coding of video blockswithin video slices. Intra-coding relies on spatial prediction to reduceor remove spatial redundancy in video within a given video frame orpicture. Inter-coding relies on temporal prediction to reduce or removetemporal redundancy in video within adjacent frames or pictures of avideo sequence. Intra-mode (I mode) may refer to any of several spatialbased compression modes. Inter-modes, such as uni-directional prediction(P mode) or bi-prediction (B mode), may refer to any of severaltemporal-based compression modes.

In the example of FIG. 2, video encoder 20 includes a partitioningmodule 35, prediction module 41, filter module 63, reference picturememory 64, summer 50, transform module 52, quantization module 54, andentropy encoding module 56. Prediction module 41 includes motionestimation module 42, motion compensation module 44, and intraprediction module 46. For video block reconstruction, video encoder 20also includes inverse quantization module 58, inverse transform module60, and summer 62. Filter module 63 is intended to represent one or moreloop filters such as a deblocking filter, an adaptive loop filter (ALF),and a sample adaptive offset (SAO) filter. Although filter module 63 isshown in FIG. 2 as being an in loop filter, in other configurations,filter module 63 may be implemented as a post loop filter.

Source device 12 or another network device, such as a MANE may providean external indication 34 to destination device 14 that causesdestination device 14 to treat a CRA picture received at the destinationdevice as a BLA picture. For example, external indication 34, which isgenerally external to the destination device 14, and generally nottransmitted as part of the bitstream, may be generated by predictionmodule 41, which may have access to indications related to the status ofthe bitstream. This is only one example, however, other units or modulesin source device 12, or in other devices external to source device 12,may also generate an external indication.

As shown in FIG. 2, video encoder 20 receives video data, andpartitioning module 35 partitions the data into video blocks. Thispartitioning may also include partitioning into slices, tiles, or otherlarger units, as wells as video block partitioning, e.g., according to aquadtree structure of LCUs and CUs. Video encoder 20 generallyillustrates the components that encode video blocks within a video sliceto be encoded. The slice may be divided into multiple video blocks (andpossibly into sets of video blocks referred to as tiles). Predictionmodule 41 may select one of a plurality of possible coding modes, suchas one of a plurality of intra coding modes or one of a plurality ofinter coding modes, for the current video block based on error results(e.g., coding rate and the level of distortion). Prediction module 41may provide the resulting intra- or inter-coded block to summer 50 togenerate residual block data and to summer 62 to reconstruct the encodedblock for use as a reference picture.

Intra prediction module 46 within prediction module 41 may performintra-predictive coding of the current video block relative to one ormore neighboring blocks in the same frame or slice as the current blockto be coded to provide spatial compression. Motion estimation module 42and motion compensation module 44 within prediction module 41 performinter-predictive coding of the current video block relative to one ormore predictive blocks in one or more reference pictures to providetemporal compression.

Motion estimation module 42 may be configured to determine theinter-prediction mode for a video slice according to a predeterminedpattern for a video sequence. The predetermined pattern may designatevideo slices in the sequence as P slices, B slices or GPB slices. Motionestimation module 42 and motion compensation module 44 may be highlyintegrated, but are illustrated separately for conceptual purposes.Motion estimation, performed by motion estimation module 42, is theprocess of generating motion vectors, which estimate motion for videoblocks. A motion vector, for example, may indicate the displacement of aPU of a video block within a current video frame or picture relative toa predictive block within a reference picture.

A predictive block is a block that is found to closely match the PU ofthe video block to be coded in terms of pixel difference, which may bedetermined by sum of absolute difference (SAD), sum of square difference(SSD), or other difference metrics. In some examples, video encoder 20may calculate values for sub-integer pixel positions of referencepictures stored in reference picture memory 64. For example, videoencoder 20 may interpolate values of one-quarter pixel positions,one-eighth pixel positions, or other fractional pixel positions of thereference picture. Therefore, motion estimation module 42 may perform amotion search relative to the full pixel positions and fractional pixelpositions and output a motion vector with fractional pixel precision.

Motion estimation module 42 calculates a motion vector for a PU of avideo block in an inter-coded slice by comparing the position of the PUto the position of a predictive block of a reference picture. Thereference picture may be selected from a first reference picture list(List 0) or a second reference picture list (List 1), each of whichidentify one or more reference pictures stored in reference picturememory 64. Motion estimation module 42 sends the calculated motionvector to entropy encoding module 56 and motion compensation module 44.

Motion compensation, performed by motion compensation module 44, mayinvolve fetching or generating the predictive block based on the motionvector determined by motion estimation, possibly performinginterpolations to sub-pixel precision. Upon receiving the motion vectorfor the PU of the current video block, motion compensation module 44 maylocate the predictive block to which the motion vector points in one ofthe reference picture lists. Video encoder 20 forms a residual videoblock by subtracting pixel values of the predictive block from the pixelvalues of the current video block being coded, forming pixel differencevalues. The pixel difference values form residual data for the block,and may include both luma and chroma difference components. Summer 50represents the component or components that perform this subtractionoperation. Motion compensation module 44 may also generate syntaxelements associated with the video blocks and the video slice for use byvideo decoder 30 in decoding the video blocks of the video slice.

Intra-prediction module 46 may intra-predict a current block, as analternative to the inter-prediction performed by motion estimationmodule 42 and motion compensation module 44, as described above. Inparticular, intra-prediction module 46 may determine an intra-predictionmode to use to encode a current block. In some examples,intra-prediction module 46 may encode a current block using variousintra-prediction modes, e.g., during separate encoding passes, andintra-prediction module 46 (or mode select module 40, in some examples)may select an appropriate intra-prediction mode to use from the testedmodes. For example, intra-prediction module 46 may calculaterate-distortion values using a rate-distortion analysis for the varioustested intra-prediction modes, and select the intra-prediction modehaving the best rate-distortion characteristics among the tested modes.Rate-distortion analysis generally determines an amount of distortion(or error) between an encoded block and an original, unencoded blockthat was encoded to produce the encoded block, as well as a bit rate(that is, a number of bits) used to produce the encoded block.Intra-prediction module 46 may calculate ratios from the distortions andrates for the various encoded blocks to determine which intra-predictionmode exhibits the best rate-distortion value for the block.

In any case, after selecting an intra-prediction mode for a block,intra-prediction module 46 may provide information indicative of theselected intra-prediction mode for the block to entropy coding module56. Entropy coding module 56 may encode the information indicating theselected intra-prediction mode in accordance with the techniques of thisdisclosure. Video encoder 20 may include configuration data in thetransmitted bitstream. The configuration data may include a plurality ofintra-prediction mode index tables and a plurality of modifiedintra-prediction mode index tables (also referred to as codeword mappingtables), definitions of encoding contexts for various blocks, andindications of a most probable intra-prediction mode, anintra-prediction mode index table, and a modified intra-prediction modeindex table to use for each of the contexts.

After prediction module 41 generates the predictive block for thecurrent video block via either inter-prediction or intra-prediction,video encoder 20 forms a residual video block by subtracting thepredictive block from the current video block. The residual video datain the residual block may be included in one or more TUs and applied totransform module 52. Transform module 52 transforms the residual videodata into residual transform coefficients using a transform, such as adiscrete cosine transform (DCT) or a conceptually similar transform.Transform module 52 may convert the residual video data from a pixeldomain to a transform domain, such as a frequency domain.

Transform module 52 may send the resulting transform coefficients toquantization module 54. Quantization module 54 quantizes the transformcoefficients to further reduce bit rate. The quantization process mayreduce the bit depth associated with some or all of the coefficients.The degree of quantization may be modified by adjusting a quantizationparameter. In some examples, quantization module 54 may then perform ascan of the matrix including the quantized transform coefficients.Alternatively, entropy encoding module 56 may perform the scan.

Following quantization, entropy encoding module 56 entropy encodes thequantized transform coefficients. For example, entropy encoding module56 may perform context adaptive variable length coding (CAVLC), contextadaptive binary arithmetic coding (CABAC), syntax-based context-adaptivebinary arithmetic coding (SBAC), probability interval partitioningentropy (PIPE) coding or another entropy encoding methodology ortechnique. Following the entropy encoding by entropy encoding module 56,the encoded bitstream may be transmitted to video decoder 30, orarchived for later transmission or retrieval by video decoder 30.Entropy encoding module 56 may also entropy encode the motion vectorsand the other syntax elements for the current video slice being coded.

Inverse quantization module 58 and inverse transform module 60 applyinverse quantization and inverse transformation, respectively, toreconstruct the residual block in the pixel domain for later use as areference block of a reference picture. Motion compensation module 44may calculate a reference block by adding the residual block to apredictive block of one of the reference pictures within one of thereference picture lists. Motion compensation module 44 may also applyone or more interpolation filters to the reconstructed residual block tocalculate sub-integer pixel values for use in motion estimation. Summer62 adds the reconstructed residual block to the motion compensatedprediction block produced by motion compensation module 44 to produce areference block for storage in reference picture memory 64. Thereference block may be used by motion estimation module 42 and motioncompensation module 44 as a reference block to inter-predict a block ina subsequent video frame or picture.

Video encoder 20 of FIG. 2 represents an example of a video encoder thatmay be configured to signal that a clean random access (CRA) pictureshould be treated as a broken link access (BRA) picture, as describedherein.

FIG. 3 is a block diagram illustrating an example video decoder 30 thatmay implement the techniques of this disclosure, which generally relateto treating a CRA picture as a BLA picture based on an externalindication 70, which may be generated by a network entity 29, such as aMANE or some other external device (not shown). In one example, a videodecoder 30 receives an external indication 70 that a flag 74 should beset. The external indication 70 is received by video decoder 30. Inother examples, external indication 70 may be received and processedexternal to video decoder 30. External indication processing unit 72sets the flag 74 based on the external indication. The flag is thenpassed to prediction module 81. In the illustrated example, externalindication processing unit 72 is within video decoder 30. In otherexamples, external indication processing unit 72 may be external to andseparate from video decoder 30. At a video decoder 30, prediction module(81) checks the flag and when the flag is set treats one clean randomaccess (CRA) picture as a broken link access (BLA) picture.

In some examples, a default value of the flag is “0” and a set value ofthe flag is “1.” In other examples, the opposite may be true, thedefault value of the flag is “1” and a set value of the flag is “0.” Inother words, the flag may be active high (“1”) or active low (“0”).

In some examples, when decoding a coded slice Network Abstraction Layer(NAL) unit, if the first flag is set, prediction module 81 may change aNAL unit type of the NAL unit. When decoding the coded slice NetworkAbstraction Layer (NAL) unit, if the first flag is set, predictionmodule 81 may change a value of a second flag. The second flag may bethe no_output_of_prior_pics flag. Additionally, when decoding the codedslice Network Abstraction Layer (NAL) unit, if the flag is set, theprediction module may set the value of a second flag to “1.”

In an example, when a current picture is a CRA picture, and when someexternal indication is available to set a variable indicating that a CRApicture should be handled as a BLA picture (e.g., HandleCraAsBlaFlag)then the variable (e.g., HandleCraAsBlaFlag) may be set to the valueprovided by the external means. Otherwise, the value of the variable(e.g., HandleCraAsBlaFlag) may be set to indicate that the CRA pictureis not to be handled as a BRA picture. For example, theHandleCraAsBlaFlag may be set to “1” to indicate that the CRA picture isto be handled as a BRA picture and set to “0” to indicate that the CRApicture is not to be handled as a BRA picture.

It should be noted that, while some examples of external indication maybe described herein, these are not intended to be an exhaustive list.Many possible external indications could be used.

In some examples, when the current picture is a CRA picture and variableindicating that a CRA picture should be handled as a BLA picture (e.g.,HandleCraAsBlaFlag) is equal to “1,” where “1” indicates that the CRApicture should be handled as a BLA picture, the value ofno_output_of_prior_pics_flag may be set to “1,” and the followingapplies during the parsing and decoding processes for each coded slicesegment NAL unit

In an example, the no_output_of_prior_pics_flag specifies how thepreviously-decoded pictures in the decoded picture buffer are treatedafter decoding of an IDR or a BLA picture. In an example, when the IDRor BLA picture is the first picture in the bitstream, the value ofno_output_of_prior_pics_flag has no effect on the decoding process. Whenthe IDR or BLA picture is not the picture in the bitstream and the valueof pic_width_in_luma_samples or pic_height_in_luma_samples orsps_max_dec_pic_buffering[sps_max_temporal_layers_minus1] derived fromthe active sequence parameter set is different from the value ofpic_width_in_luma_samples or pic_height_in_luma_samples orsps_max_dec_pic_buffering [sps_max_temporal_layers_minus1] derived fromthe sequence parameter set active for the preceding picture,no_output_of_prior_pics_flag equal to “1” may (but should not) beinferred by the decoder, regardless of the actual value ofno_output_of_prior_pics_flag.

In the example of FIG. 3, video decoder 30 includes an entropy decodingmodule 80, prediction module 81, inverse quantization module 86, inversetransformation module 88, summer 90, filter module 91, and referencepicture memory 92. Prediction module 81 includes motion compensationmodule 82 and intra prediction module 84. Video decoder 30 may, in someexamples, perform a decoding pass generally reciprocal to the encodingpass described with respect to video encoder 20 from FIG. 2.

During the decoding process, video decoder 30 receives an encoded videobitstream that represents video blocks of an encoded video slice andassociated syntax elements from video encoder 20. Video decoder 30 mayreceive the encoded video bitstream from a network entity 29. Networkentity 29 may, for example, be a server, a MANE, a video editor/splicer,or other such device configured to implement one or more of thetechniques described above. As described above, some of the techniquesdescribed in this disclosure may be implemented by network entity 29prior to network entity 29 transmitting the encoded video bitstream tovideo decoder 30. In some video decoding systems, network entity 29 andvideo decoder 30 may be parts of separate devices, while in otherinstances, the functionality described with respect to network entity 29may be performed by the same device that comprises video decoder 30.

As discussed above, a network device, such as network entity 29, whichmay be a MANE may provide an external indication 34 to destinationdevice 14 that causes destination device 14 to treat a CRA picturereceived at the destination device as a BLA picture. For example,external indication 34, which is generally external to the destinationdevice 14, and generally not transmitted as part of the bitstream, maybe generated by prediction module 41, which may have access toindications related to the status of the bitstream. This is only oneexample, however, other units or modules in source device 12, or inother devices external to source device 12, may also generate anexternal indication.

Entropy decoding module 80 of video decoder 30 entropy decodes thebitstream to generate quantized coefficients, motion vectors, and othersyntax elements. Entropy decoding module 80 forwards the motion vectorsand other syntax elements to prediction module 81. Video decoder 30 mayreceive the syntax elements at the video slice level and/or the videoblock level.

When the video slice is coded as an intra-coded (I) slice, intraprediction module 84 of prediction module 81 may generate predictiondata for a video block of the current video slice based on a signaledintra prediction mode and data from previously decoded blocks of thecurrent frame or picture. When the video frame is coded as aninter-coded (i.e., B, P or GPB) slice, motion compensation module 82 ofprediction module 81 produces predictive blocks for a video block of thecurrent video slice based on the motion vectors and other syntaxelements received from entropy decoding module 80. The predictive blocksmay be produced from one of the reference pictures within one of thereference picture lists. Video decoder 30 may construct the referenceframe lists, List 0 and List 1, using default construction techniquesbased on reference pictures stored in reference picture memory 92.

Motion compensation module 82 determines prediction information for avideo block of the current video slice by parsing the motion vectors andother syntax elements, and uses the prediction information to producethe predictive blocks for the current video block being decoded. Forexample, motion compensation module 82 may use some of the receivedsyntax elements to determine a prediction mode (e.g., intra- orinter-prediction) used to code the video blocks of the video slice, aninter-prediction slice type (e.g., B slice, P slice, or GPB slice).Additionally, motion compensation module 82 may use constructioninformation for one or more of the reference picture lists for theslice, motion vectors for each inter-encoded video block of the slice,inter-prediction status for each inter-coded video block of the slice,and other information to decode the video blocks in the current videoslice.

Motion compensation module 82 may also perform interpolation based oninterpolation filters. Motion compensation module 82 may useinterpolation filters as used by video encoder 20 during encoding of thevideo blocks to calculate interpolated values for sub-integer pixels ofreference blocks. In this case, motion compensation module 82 maydetermine the interpolation filters used by video encoder 20 from thereceived syntax elements and use the interpolation filters to producepredictive blocks.

Inverse quantization module 86 inverse quantizes, i.e., de-quantizes,the quantized transform coefficients provided in the bitstream anddecoded by entropy decoding module 80. The inverse quantization processmay include use of a quantization parameter calculated by video encoder20 for each video block in the video slice to determine a degree ofquantization and, likewise, a degree of inverse quantization that shouldbe applied. Inverse transform module 88 applies an inverse transform,e.g., an inverse DCT, an inverse integer transform, or a conceptuallysimilar inverse transform process, to the transform coefficients inorder to produce residual blocks in the pixel domain.

After motion compensation module 82 generates the predictive block forthe current video block based on the motion vectors and other syntaxelements, video decoder 30 forms a decoded video block by summing theresidual blocks from inverse transform module 88 with the correspondingpredictive blocks generated by motion compensation module 82. Summer 90represents the component or components that perform this summationoperation. If desired, loop filters (either in the coding loop or afterthe coding loop) may also be used to smooth pixel transitions, orotherwise improve the video quality. Filter module 91 is intended torepresent one or more loop filters such as a deblocking filter, anadaptive loop filter (ALF), and a sample adaptive offset (SAO) filter.Although filter module 91 is shown in FIG. 3 as being an in loop filter,in other configurations, filter module 91 may be implemented as a postloop filter. The decoded video blocks in a given frame or picture arethen stored in reference picture memory 92, which stores referencepictures used for subsequent motion compensation. Reference picturememory 92 also stores decoded video for later presentation on a displaydevice, such as display device 32 of FIG. 1.

Video decoder 30 of FIG. 3 represents an example of a video decoderconfigured to treat one clean random access (CRA) pictures as a brokenlink access (BRA) picture, as described herein.

FIG. 4 is a block diagram illustrating an example set of devices thatform part of network 100. In this example, network 100 includes routingdevices 104A, 104B (routing devices 104) and transcoding device 106.Routing devices 104 and transcoding device 106 are intended to representa small number of devices that may form part of network 100. Othernetwork devices, such as switches, hubs, gateways, firewalls, bridges,and other such devices may also be included within network 100.Moreover, additional network devices may be provided along a networkpath between server device 102 and client device 108. Server device 102may correspond to source device 12 (FIG. 1), while client device 108 maycorrespond to destination device 14 (FIG. 1), in some examples.Accordingly, server device 102 generally does not receive the externalindication to treat a CRA picture as a BLA picture. Server 102 mayprovide an external indication 34 to client device 108 that causesclient device 108 to treat a CRA picture received at the destinationdevice as a BLA picture, however. Similarly, routing devices 104A, 104B(routing devices 104) and transcoding device 106 generally do notreceive the external indication to treat a CRA picture as a BLA picture,but may provide an external indication 34 to client device 108 thatclient device 108 to treat a CRA picture received at the destinationdevice as a BLA picture, however. Some examples described herein mayinclude one or more of the following: network devices, servers MANEs,hypertext transfer protocol (HTTP) caches, or web proxies.

In some examples client device 108 may set the flag after receiving amessage that a change in bit rate of a bit stream has occurred.Accordingly, the client device may set the flag based on the change ofthe bit rate. In some examples, a decoder in client device 108 maydecode a coded slice NAL unit. A prediction module in a decoder inclient device 108 may parse each coded slice NAL unit to identify theNAL unit type. Additionally, the prediction module may decode the codedslice NAL unit based on the NAL unit type.

In general, routing devices 104 implement one or more routing protocolsto exchange network data through network 100. In some examples, routingdevices 104 may be configured to perform proxy or cache operations.Therefore, in some examples, routing devices 104 may be referred to asproxy devices. In general, routing devices 104 execute routing protocolsto discover routes through network 100. By executing such routingprotocols, routing device 104B may discover a network route from itselfto server device 102 via routing device 104A.

The techniques of this disclosure may be implemented by network devicessuch routing devices 104 and transcoding device 106, but also may beimplemented by client device 108. In this manner, routing devices 104,transcoding device 106, and client device 108 represent examples ofdevices configured to perform the techniques of this disclosure,including techniques recited in the CLAIMS portion of this disclosure.Moreover, the devices of FIG. 1 and encoder shown in FIG. 2 and thedecoder shown in FIG. 3 are also exemplary devices that can beconfigured to perform the techniques of this disclosure, includingtechniques recited in the CLAIMS portion of this disclosure.

FIG. 5 is a flow chart illustrating an example method in accordance withone or more examples described in this disclosure. In an example, videodecoder 30 or other device receives an external indication (500). Videodecoder 30 then treats a clean random access (CRA) picture as a brokenlink access (BLA) picture based on the external indication (502). Insome examples, the external indication indicates whether a flag shouldbe set in the video decoder. Accordingly, the video decoder may set theflag based on the external indication, as will be discussed in greaterdetail with respect to FIG. 6. The decoder may or some internalfunctionality, such as an external indication processing unit or aprediction module may then check the flag. In an example, the predictionmodule may treat the CRA picture as a BLA picture based on the externalindication that indicates the CRA picture should be treated as a BLApicture based on the flag.

FIG. 6 is a flow chart illustrating another example method in accordancewith one or more examples described in this disclosure. In theillustrated example, a video decoder 30 receives an external indication70 that a flag 74 should be set (600). In the illustrated example, theexternal indication 70 is received by video decoder 30. In otherexamples, external indication 70 may be received and processed externalto video decoder 30.

External indication processing unit 72 sets the first flag 74 based onthe external indication (602). The first flag is then passed toprediction module 81. In the illustrated example, external indicationprocessing unit 72 is within video decoder 30. In other examples,external indication processing unit 72 may be external to and separatefrom video decoder 30.

At decoder 30, prediction module 81 checks the flag and when the flag isset treats a clean random access (CRA) picture as a broken link access(BLA) picture (604). In an example, when decoding a coded slice NetworkAbstraction Layer (NAL) unit, if the first flag is equal to “1” and aNAL unit type of a coded slice NAL unit indicates a CRA picture (e.g.the value is equal to “4” or “5” per HEVC WD7) the external indicationprocessing unit 72 or other unit within video decoder 30 changes thevalue of the NAL unit type to indicate a BLA picture (e.g. increases thevalue of the NAL unit type by 2 per HEVC WD7). Additionally, theprediction module 81 sets the value of a second flag to 1. If theprevious picture in decoding order is a RAP picture and the rap_pic_idof the current slice is equal to the rap_pic_id of the previous picture,the following applies. If the next picture in decoding order is not aRAP picture, change the value of rap_pic_id of a current slice to bedifferent from a rap_pic_id of the previous picture in decoding order.Otherwise, the value of the rap_pic_id of the current picture is changedto be a value that is different from the rap_pic_id of both the previouspicture and the next picture in decoding order.

In another example, at video decoder 30, prediction module 81 or anotherunit within video decoder 30 decodes a coded slice NAL unit. If thefirst flag is equal to “1” and a NAL unit type of the coded slice NALunit indicates a CRA picture (e.g. the value is equal to “4” or “5” perHEVC WD7) the prediction module (or other unit) changes the value of theNAL unit type to indicate a BLA picture (e.g. increases the value of theNAL unit type by 2 per HEVC WD7). Possibly additionally, if picturetiming SEI messages are present and DPB output times for all pictures ina DPB are smaller than the DPB output time of a current picture, theprediction module 81 or other unit sets a value of a second flag to 1.Otherwise, if the value of the second flag is set to “0” and if aprevious picture in decoding order is a RAP picture and a rap_pic_id ofa current slice is equal to the rap_pic_id of the previous picture, thefollowing applies. If the next picture in decoding order is not the RAPpicture, the prediction module 81 or other unit changes a value ofrap_pic_id of a current slice to be different from a rap_pic_id of theprevious picture in decoding order. Otherwise, the value of therap_pic_id of the prediction module or other unit changes the currentpicture to be a value that is different from the rap_pic_id of both theprevious picture and the next picture in decoding order.

FIG. 7 is a flowchart illustrating an example method in accordance withone or more examples described in this disclosure. A device, such as anetwork device, e.g., a MANE, receives a bitstream including a CRApicture (700). The network device determines that the CRA picture shouldbe treated as a BLA Picture (702). For example, the network device maydetermine that a CRA picture should be treated as a BLA picture toenable the output and/or display of more picture when output of decodedpictures earlier in decoding order than an IDR or BLA picture are alldiscarded after decoding of the IDR or BLA picture without output and/ordisplay. Sometimes displaying more of those pictures may provide betteruser experience. Accordingly, the network device transmits the CRApicture and an external indication that the CRA Picture should beconverted to a BLA Picture (704).

FIG. 8 is a flowchart illustrating an exemplary operation of a firstdevice sending an external indication and responsive actions of a seconddevice receiving the external indication. A source device, such as anetwork device, e.g., a MANE, receives a bitstream including a CRApicture (800). The network device determines that the CRA picture shouldbe treated as a BLA Picture (802). Accordingly, the network devicetransmits the CRA picture and an external indication that the CRAPicture should be converted to a BLA Picture (804). A video decoder 30receives the CRA picture and the external indication 70 that the CRApicture should be converted to a BLA picture (806).

External indication processing unit 72 sets a flag 74 based on theexternal indication (808). The flag may then be passed to predictionmodule 81. In an example, external indication processing unit 72 iswithin video decoder 30. In other examples, external indicationprocessing unit 72 may be external to and separate from video decoder30. At decoder 30, prediction module 81 checks the flag and when theflag is set treats the CRA pictures as a BLA picture (810).

In the example of FIG. 8, a flag is used to indicate that an externalindication has been received. In other examples, similar to FIG. 5,video decoder 30 or other device receives an external indication andthen treat the CRA picture as a BLA picture based on the externalindication.

In one example, a decoder changes a CRA picture to a BLA picture as afunction at the decoder side. In reception or inference of such amessage, one function of the decoder side may perform the change of theidentified CRA picture to a BLA picture of the bitstream, before thecoded picture is sent to the decoder for decoding.

A CRA picture may be changed to a BLA picture. For each coded slice NALunit, if nal_unit_type indicates a CRA picture, e.g. the value is equalto “4” or “5” per HEVC WD7, the following applies, (1) the value ofnal_unit_type is changed to indicate a BLA picture, e.g., the value isincreased by 2, (2) the value of no_output_of_prior_pics_flag is set to1, (3) if the previous picture in decoding order is a RAP picture andthe rap_pic_id of the current slice is equal to the rap_pic_id of theprevious picture, the following applies: (a) if the next picture indecoding order is not a RAP picture, the value of rap_pic_id of thecurrent slice is changed to be different than the rap_pic_id of theprevious picture in decoding order, but still in the allowed value rangeof the syntax element, or (b) otherwise (the next picture in decodingorder is a RAP picture), the value of rap_pic_id of the current pictureis changed to be a value that is different than the rap_pic_id of boththe previous picture and the next picture in decoding order, but stillin the allowed value range of the syntax element.

Handling a CRA picture as a CRA picture that starts a bitstream will nowbe described. An indication that a particular CRA picture should behandled as a BLA picture, as described above, may also be changed to orinterpreted as an indication that a particular CRA picture should behandled as a CRA picture that is the first picture in a bitstream,provided that the changes described below are made to the HEVC draftspecification.

In an example, the variable CraIsFirstPicFlag is associated with eachCRA picture. The value of CraIsFirstPicFlag for some CRA pictures may bespecified by external means. If a CRA picture is the first picture inthe bitstream, then the value of CraIsFirstPicFlag for the CRA pictureis set to 1, regardless of the value indicated by the externalindication (when present). Otherwise, when the value ofCraIsFirstPicFlag for the CRA picture is not specified by externalmeans, it is set to “0.”

When decoding (including parsing) each coded slice NAL unit, ifCraIsFirstPicFlag is equal to “1” and nal_unit_type is equal to “4” or5, the value of no_output_of_prior_pics_flag may be set to 1. If theprevious picture in decoding order is a RAP picture and the rap_pic_idof the current slice is equal to the rap_pic_id of the previous picture,then, if the next picture in decoding order is not a RAP picture, thevalue of rap_pic_id of the current slice is changed to be different thanthe rap_pic_id of the previous picture in decoding order, but still inthe allowed value range of the syntax element. Otherwise (the nextpicture in decoding order is a RAP picture), the value of rap_pic_id ofthe current picture is changed to be a value that is different than therap_pic_id of both the previous picture and the next picture in decodingorder, but still in the allowed value range of the syntax element.

Alternatively, instead of setting the value ofno_output_of_prior_pics_flag may be set to 1, the prediction module 81may, if the picture timing SEI messages are present and the DPB outputtimes for all pictures in the DPB are smaller than the DPB output timeof the current picture, the value of no_output_of_prior_pics_flag is setto 1, Otherwise if the value of no_output_of_prior_pics_flag is set to“0.”

In other examples, various definitions of picture order count, taggedfor discard (TFD) picture may be changed from the HEVC WD9 or otherworking drafts of the standard. Accordingly, the definitions providedbelow may be different from the standard. These definitions may notapply to some or all examples described herein.

In some examples, a coded video sequence is a sequence of access unitsthat may include, in decoding order, of a CRA access unit that may havea CraIsFirstPicFlag equal to 1, an IDR access unit or a BLA access unit,followed by zero or more non-IDR and non-BLA access units including allsubsequent access units up to but not including any subsequent IDR orBLA access unit.

In some examples, a picture order count is a variable that may beassociated with each coded picture and has a value that is increasingwith increasing picture position in output order relative to one of thefollowing coded pictures: (1) the previous IDR picture in decodingorder, if any (2) the previous BLA picture in decoding order, if any,and (3) the previous CRA picture in decoding order, if any and in someexamples, if the previous CRA picture has CraIsFirstPicFlag equal to 1

In some examples, if more than one of the above coded pictures ispresent, the picture order count is relative to the last of such codedpictures in decoding order. A tagged for discard (TFD) picture: A codedpicture for which each slice has nal_unit_type equal to 2; a TFD pictureis associated with the previous CRA picture or BLA picture in decodingorder and precedes the associated picture in output order; when theassociated picture is a BLA picture, or when the associated picture is aCRA picture that may have a CraIsFirstPicFlag equal to 1, the TFDpicture may not be correctly decodable and is not output.

In some examples, the semantics of no_output_of_prior_pics_flag may bechanged so that the no_output_of_prior_pics_flag specifies how thepreviously-decoded pictures in the decoded picture buffer are treatedafter decoding of a CRA picture with CraIsFirstPicFlag equal to “1” oran IDR or a BLA picture.

In some examples, when the CRA picture with CraIsFirstPicFlag equal to“1” or IDR or BLA picture is the first picture in the bitstream, thevalue of no_output_of_prior_pics_flag has no effect on the decodingprocess. When the CRA picture with CraIsFirstPicFlag equal to “1” or IDRor BLA picture is not the first picture in the bitstream and the valueof pic_width_in_luma_samples or pic_height_in_luma_samples orsps_max_dec_pic_buffering[sps_max_temporal_layers_minus1] derived fromthe active sequence parameter set is different from the value ofpic_width_in_luma_samples or pic_height_in_luma_samples orsps_max_dec_pic_buffering[sps_max_temporal_layers_minus1] derived fromthe sequence parameter set active for the preceding picture,no_output_of_prior_pics_flag equal to “1” may (but should not) beinferred by the decoder, regardless of the actual value ofno_output_of_prior_pics_flag.

In some examples, a change can be made to the following in sub-clause8.1 of HEVC WD7, e.g., change: if the first coded picture in thebitstream is a CRA picture, and the current picture is a TFD pictureassociated with the CRA picture, or if the previous RAP picturepreceding the current picture in decoding order is a BLA picture and thecurrent picture is a TFD picture associated with the BLA picture,PicOutputFlag is set equal to “0” and the decoding process forgenerating unavailable reference pictures specified in subclause 8.3.3is invoked (only needed to be invoked for one slice of a picture) to: ifa CRA picture has CraIsFirstPicFlag equal to 1, and the current pictureis a TFD picture associated with the CRA picture, or if the previous RAPpicture preceding the current picture in decoding order is a BLA pictureand the current picture is a TFD picture associated with the BLApicture, PicOutputFlag is set equal to “0” and the decoding process forgenerating unavailable reference pictures specified in subclause 8.3.3is invoked (only needed to be invoked for one slice of a picture).

In some examples, a change the following in subclause 8.3.1 of HEVC WD7can be made, e.g., change: the current picture is a CRA picture and isthe first coded picture in the bitstream to the current picture is a CRApicture with CraIsFirstPicFlag equal to 1.

In some examples, a change to the following in subclause 8.3.1 of HEVCWD7 can be made, e.g., change: if the current picture is an IDR or a BLApicture, or if the first coded picture in the bitstream is a CRA pictureand the current picture is the first coded picture in the bitstream,PicOrderCntMsb is set equal to “0.” Otherwise, PicOrderCntMsb is derivedas specified by the following pseudo-code to if the current picture isan IDR or a BLA picture, or a CRA picture with CraIsFirstPicFlag equalto 1, PicOrderCntMsb is set equal to “0.” Otherwise, PicOrderCntMsb isderived as specified by the following pseudo-code.

In some examples, a change the following in subclause 8.3.2 of HEVC WD7may be made, e.g., change NOTE 4—There may be one or more referencepictures that are included in the reference picture set but not presentin the decoded picture buffer. Entries in RefPicSetStFoll orRefPicSetLtFoll that are equal to “no reference picture” should beignored. Unless either of the following two conditions is true, anunintentional picture loss should be inferred for each entry inRefPicSetStCurrBefore, RefPicSetStCurrAfter and RefPicSetLtCurr that isequal to “no reference picture”: a) the first coded picture in thebitstream is a CRA picture and the current coded picture is a TFDpicture associated with the first coded picture in the bitstream; b) theprevious RAP picture preceding the current coded picture in decodingorder is a BLA picture and the current coded picture is a TFD pictureassociated with the BLA picture to NOTE 4—There may be one or morereference pictures that are included in the reference picture set butnot present in the decoded picture buffer. Entries in RefPicSetStFoll orRefPicSetLtFoll that are equal to “no reference picture” should beignored. Unless the previous RAP picture preceding the current codedpicture in decoding order is a CRA picture with CraIsFirstPicFlag equalto “1” or a BLA picture, and the current coded picture is a TFD pictureassociated with the previous RAP picture, an unintentional picture lossshould be inferred for each entry in RefPicSetStCurrBefore,RefPicSetStCurrAfter and RefPicSetLtCurr that is equal to “no referencepicture.”

In some examples, a change the following in subclause 8.3.2 of HEVC WD7may be made, e.g., change: Unless either of the following conditions istrue, there shall be no entry in RefPicSetStCurrBefore,RefPicSetStCurrAfter or RefPicSetLtCurr that is equal to “no referencepicture”: a) the first coded picture in the bitstream is a CRA pictureand the current coded picture is a TFD picture associated with the firstcoded picture in the bitstream; b) the previous RAP picture precedingthe current coded picture in decoding order is a BLA picture and thecurrent coded picture is a TFD picture associated with the BLA pictureto Unless the previous RAP picture preceding the current coded picturein decoding order is a CRA picture with CraIsFirstPicFlag equal to “1”or a BLA picture, and the current coded picture is a TFD pictureassociated with the previous RAP picture, there shall be no entry inRefPicSetStCurrBefore, RefPicSetStCurrAfter or RefPicSetLtCurr that isequal to “no reference picture.”

In some examples, a change to the first three paragraphs in subclause8.3.3.1 of HEVC WD7 may be made as follows: this process is invoked onceper coded picture, after the invocation of the decoding process forreference picture set as specified in subclause 8.3.2, when the previousRAP picture preceding the current coded picture in decoding order is aCRA picture with CraIsFirstPicFlag equal to “1” or a BLA picture, andthe current coded picture is a TFD picture associated with the previousRAP picture. NOTE 1—The entire specification herein of the decodingprocess for TFD pictures associated with a CRA picture at the beginningof the bitstream or for TFD pictures associated with a BLA picture isonly included for purposes of specifying constraints on the allowedsyntax content of such pictures. In actual decoders, any TFD picturesassociated with a CRA picture at the beginning of the bitstream or anyTFD pictures associated with a BLA picture may simply be ignored(removed from the bitstream and discarded), as they are not specifiedfor output and have no effect on the decoding process of any otherpictures that are specified for output. When the previous RAP picturepreceding the current coded picture in decoding order is a CRA picturewith CraIsFirstPicFlag equal to “1” or a BLA picture, and the currentcoded picture is a TFD picture associated with the previous RAP picture,the following applies.

In some examples, a change the following in subclause C.4 of HEVC WD7may be made, e.g., change: NOTE 1—This constraint guaranteesdecodability of a TFD picture if its associated RAP picture is a CRApicture and if that CRA picture is not the first coded picture in thebitstream to NOTE 1—This constraint guarantees decodability of a TFDpicture if its associated RAP picture is a CRA picture and if that CRApicture has CraIsFirstPicFlag equal to “0.”

In some examples, a change to the third paragraph in subclause C.3.1 ofHEVC WD7 may be made as follows: If the current picture is a CRA picturewith CraIsFirstPicFlag equal to “1” or an IDR or a BLA picture, thefollowing applies when the CRA picture with CraIsFirstPicFlag equal to“1” or IDR or BLA picture is not the picture decoded and the value ofpic_width_in_luma_samples or pic_height_in_luma_samples orsps_max_dec_pic_buffering[i] for any possible value of i derived fromthe active sequence parameter set is different from the value ofpic_width_in_luma_samples or pic_height_in_luma_samples orsps_max_dec_pic_buffering[i] derived from the sequence parameter setthat was active for the preceding picture, respectively,no_output_of_prior_pics_flag is inferred to be equal to “1” by the HRD,regardless of the actual value of no_output_of_prior_pics_flag. NOTE1—Decoder implementations should try to handle picture or DPB sizechanges more gracefully than the HRD in regard to changes inpic_width_in_luma_samples, pic_height_in_luma_samples, orsps_max_dec_pic_buffering[i]. When no_output_of_prior_pics_flag is equalto “1” or is inferred to be equal to 1, all picture storage buffers inthe DPB are emptied without output of the pictures they contain, and DPBfullness is set to “0.”

In some examples, a change to the entire subclause C.5.2 of HEVC WD7 maybe made as follows, the removal of pictures from the DPB before decodingof the current picture (but after parsing the slice header of the firstslice of the current picture) happens instantaneously when the firstdecoding unit of the access unit containing the current picture isremoved from the CPB and proceeds as follows. The decoding process forreference picture set as specified in subclause 8.3.2 is invoked. If thecurrent picture is a CRA picture with CraIsFirstPicFlag equal to “1” oran IDR or a BLA picture, the following applies. When the CRA picturewith CraIsFirstPicFlag equal to “1” or IDR or BLA picture is not thefirst picture decoded and the value of pic_width_in_luma_samples orpic_height_in_luma_samples or sps_max_dec_pic_buffering[i] for anypossible value of i derived from the active sequence parameter set isdifferent from the value of pic_width_in_luma_samples orpic_height_in_luma_samples or sps_max_dec_pic_buffering[i] derived fromthe sequence parameter set that was active for the preceding picture,respectively, no_output_of_prior_pics_flag is inferred to be equal to“1” by the HRD, regardless of the actual value ofno_output_of_prior_pics_flag. NOTE—Decoder implementations should try tohandle picture or DPB size changes more gracefully than the HRD inregard to changes in pic_width_in_luma_samples,pic_height_in_luma_samples or sps_max_dec_pic_buffering[i]. Whenno_output_of_prior_pics_flag is equal to “1” or is inferred to be equalto 1, all picture storage buffers in the DPB are emptied without outputof the pictures they contain. Otherwise, picture storage bufferscontaining a picture which are marked as “not needed for output” and“unused for reference” are emptied (without output). When one or more ofthe following conditions are true, the “bumping” process specified insubclause C.5.2.1 is invoked repeatedly until there is an empty picturestorage buffer to store the current decoded picture. The number ofpictures in the DPB that are marked as “needed for output” is greaterthan sps_num_reorder_pics[temporal_id]. The number of pictures in theDPB with temporal_id lower than or equal to the temporal_id of thecurrent picture is equal to sps_max_dec_pic_buffering[temporal_id].

In some examples, when the current picture is a CRA picture withCraIsFirstPicFlag equal to “1” or an IDR or a BLA picture for whichno_output_of_prior_pics_flag is not equal to “1” and is not inferred tobe equal to 1, the following two steps are performed. Picture storagebuffers containing a picture that is marked as “not needed for output”and “unused for reference” are emptied (without output). All non-emptypicture storage buffers in the DPB are emptied by repeatedly invokingthe “bumping” process specified in subclause C.5.2.1.

Some examples may include a “bumping” process. The “bumping” process maybe invoked in the following cases: (1) the current picture is a CRApicture with CraIsFirstPicFlag equal to “1” or an IDR or a BLA pictureand no_output_of_prior_pics_flag is not equal to “1” and is not inferredto be equal to 1, as specified in subclause C.5.2, (2) the number ofpictures in the DPB that are marked “needed for output” is greater thansps_num_reorder_pics[temporal_id], as specified in subclause C.5.2, and(3) the number of pictures in the DPB with temporal_id lower than orequal to the temporal_id of the current picture is equal tosps_max_dec_pic_buffering[temporal_id], as specified in subclause C.5.2.

The “bumping” process may include the following ordered steps: (1) thepicture that is first for output is selected as the one having thesmallest value of PicOrderCntVal of all pictures in the DPB marked as“needed for output,” (2) the picture is cropped, using the croppingrectangle specified in the active sequence parameter set for thepicture, the cropped picture is output, and the picture is marked as“not needed for output,” (3) if the picture storage buffer that includedthe picture that was cropped and output contains a picture marked as“unused for reference”, the picture storage buffer is emptied.

In some examples, with the above changes to the HEVC draftspecification, it may be possible to further remove all texts forsupport of BLA pictures.

An improved output of pictures will now be described. In an example, itis proposed that no_output_of_prior_pics_flag is changed tooutput_all_prior_pics_flag, this flag equal to “1” has the equivalentmeaning when no_output_of_prior_pics_flag is equal to “0.” When thisflag is equal to “0,” furthermore, the number of prior pictures that maybe used for output/display is signalled as num_output_pics.num_output_pics may be signalled as u(v), this syntax element is in therange of 0 to MaxDpbSize, exclusive. The num_output_pics pictures to beoutput/displayed are the pictures having closer display orders to theBLA or IDR picture and in the first bitstream. num_output_pics may berelated to the number of leading pictures that do not need to beoutputted.

Alternatively, num_output_pics may be signalled as ue(v). Alternatively,no_output_of_prior_pics_flag, output_all_prior_pics_flag, ornum_output_pics is not signalled and num_prior_discard_pics is directlysignalled as u(v) or ue(v) num_prior_discard_pics is in the range of 0to MaxDpbSize, exclusive. It indicates the number of the prior picturesto be discarded. The num_prior_discard_pics pictures to be discarded(thus not displayed) are the pictures having farther display orders tothe BLA or IDR picture and in the first bitstream.

Alternately, an SEI message may be added during the splicing to indicatethe additional memory, in terms of number of the frames in the firstbitstream, required to display all the pictures in the first bitstreamwhich haven't been displayed.

Signaling of picture timing will now be described. The indication of oneor more of different timing information, e.g. the earliest presentationtime (i.e., the earliest DPB output time) and the smallest picture ordercount value of all TFD pictures associated with one BLA or CRA picture,may be included in the bitstream. The information may be included in oneor more of the slice header and an SEI message (e.g. the recovery pointSEI message or buffering period SEI message or picture timing SEImessage). One or more of the following syntax elements may be includedin the slice header of a RAP picture or an SEI message associated with aRAP picture to signal the information: (1)delta_earliest_presentation_time, indicating the difference between theDPB output time of the RAP picture and the earliest DPB output time ofany picture when the RAP picture is the first picture in the bitstream(i.e., the earliest DPB output time of all DLPs associated with the RAPpicture), in units of clock ticks as specified in Annex C of HEVC WD7.The syntax element may be u(v) coded, and the number of bits used torepresent the syntax element is cpb_removal_delay_length_minus1+1 bits.The value “0” indicates that the RAP picture has no associated DLPs, (2)delta_earliest_poc, indicating the difference between the PicOrderCntValvalue of the RAP picture and the smallest PicOrderCntVal value of anypicture when the RAP picture is the first picture in the bitstream(i.e., the smallest earliest PicOrderCntVal value of all DLPs associatedwith the RAP picture). The syntax element may be ue(v) coded, and thevalue range may be 0 to MaxPicOrderCntLsb/2−1, inclusive.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored on or transmitted over, as oneor more instructions or code, a computer-readable medium and executed bya hardware-based processing unit. Computer-readable media may includecomputer-readable storage media, which corresponds to a tangible mediumsuch as data storage media, or communication media including any mediumthat facilitates transfer of a computer program from one place toanother, e.g., according to a communication protocol. In this manner,computer-readable media generally may correspond to (1) tangiblecomputer-readable storage media which is non-transitory or (2) acommunication medium such as a signal or carrier wave. Data storagemedia may be any available media that may be accessed by one or morecomputers or one or more processors to retrieve instructions, codeand/or data structures for implementation of the techniques described inthis disclosure. A computer program product may include acomputer-readable medium.

In some examples, either the message or the syntax element includes oneof the following: (1) a delta_earliest_presentation_time, indicating atime difference associated with the one or more CRA treated as BRApictures; or (2) a delta_earliest_poc, indicating a difference inpicture order value associated with the one or more CRA treated as BRApictures.

In still other examples, this disclosure contemplates a computerreadable medium comprising a data structure stored thereon, wherein thedata structure includes an encoded bitstream consistent with thisdisclosure. In particular, the data structures may include the NAL unitdesigns described herein.

In an example, a CRA picture may be treated as BRA pictures. The videodecoder 30 may change a value of a network abstraction layer (NAL) type,set a value that controls output of prior pictures, and change a pictureidentification (ID) value associated with a next picture. Video decoder30 may receive a syntax element to indicate a switching operation. Thesyntax element or message that is part of a compressed bitstream and theswitching operation instructs a decoder to treat one or more CRApictures as BRA pictures. The decoder may then decode the bitstreambased in part on the syntax element.

In an example, a video encoder 20 may generate a syntax element or amessage to indicate a switching operation. The switching operationinstructs a decoder to treat one or more CRA pictures as BRA pictures.Video encoder 20 may send the syntax element to a decoding device aspart of a compressed bitstream.

By way of example, and not limitation, such computer-readable storagemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage, or other magnetic storage devices, flashmemory, or any other medium that can be used to store desired programcode in the form of instructions or data structures and that can beaccessed by a computer. Also, any connection is properly termed acomputer-readable medium. For example, if instructions are transmittedfrom a website, server, or other remote source using a coaxial cable,fiber optic cable, twisted pair, digital subscriber line (DSL), orwireless technologies such as infrared, radio, and microwave, then thecoaxial cable, fiber optic cable, twisted pair, DSL, or wirelesstechnologies such as infrared, radio, and microwave are included in thedefinition of medium. It should be understood, however, thatcomputer-readable storage media and data storage media do not includeconnections, carrier waves, signals, or other transient media, but areinstead directed to non-transient, tangible storage media. Disk anddisc, as used herein, includes compact disc (CD), laser disc, opticaldisc, digital versatile disc (DVD), floppy disk and Blu-ray disc, wheredisks usually reproduce data magnetically, while discs reproduce dataoptically with lasers. Combinations of the above should also be includedwithin the scope of computer-readable media.

Instructions may be executed by one or more processors, such as one ormore digital signal processors (DSPs), general purpose microprocessors,application specific integrated circuits (ASICs), field programmablelogic arrays (FPGAs), or other equivalent integrated or discrete logiccircuitry. Accordingly, the term “processor,” as used herein may referto any of the foregoing structure or any other structure suitable forimplementation of the techniques described herein. In addition, in someaspects, the functionality described herein may be provided withindedicated hardware and/or software modules configured for encoding anddecoding, or incorporated in a combined codec. Also, the techniquescould be fully implemented in one or more circuits or logic elements.

The techniques of this disclosure may be implemented in a wide varietyof devices or apparatuses, including a wireless handset, an integratedcircuit (IC) or a set of ICs (e.g., a chip set). Various components,modules, or units are described in this disclosure to emphasizefunctional aspects of devices configured to perform the disclosedtechniques, but do not necessarily require realization by differenthardware units. Rather, as described above, various units may becombined in a codec hardware unit or provided by a collection ofinteroperative hardware units, including one or more processors asdescribed above, in conjunction with suitable software and/or firmware.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. A method of processing video data, the methodcomprising: receiving an external indication at a video decoder; andtreating a clean random access (CRA) picture as a broken link access(BLA) picture based on the external indication.
 2. The method of claim1, wherein the external indication indicates whether a flag should beset in the video decoder, the method further comprising: setting theflag based on the external indication; and checking the flag, whereintreating the CRA picture as a BLA picture based on the externalindication comprises treating the CRA picture as a BLA picture based onthe flag.
 3. The method of claim 2, wherein the flag is associated withthe CRA picture.
 4. The method of claim 2, wherein a default value ofthe flag is “0” and a set value of the flag is “1.”
 5. The method ofclaim 2, further comprising, when decoding a coded slice NetworkAbstraction Layer (NAL) unit, if the flag is set, changing a NAL unittype of the NAL unit.
 6. The method of claim 5, wherein the flagcomprises a first flag, the method further comprising, when decoding thecoded slice Network Abstraction Layer (NAL) unit, if the first flag isset, changing a value of a second flag, the second flag controlling theoutput of prior pictures.
 7. The method of claim 5, wherein the flagcomprises a first flag, the method further comprising, when decoding thecoded slice Network Abstraction Layer (NAL) unit, if the first flag isset, setting the value of a second flag to
 1. 8. The method of claim 5,wherein decoding a coded slice NAL unit includes parsing each codedslice NAL unit to identify a NAL unit type.
 9. The method of claim 8,wherein the flag comprises a first flag and when decoding a coded sliceNetwork Abstraction Layer (NAL) unit, if the first flag is equal to 1and a NAL unit type of a coded slice NAL unit indicates a CRA picture:changing the value of the NAL unit type to indicate a BLA picture. 10.The method of claim 2, wherein setting the flag further comprisesreceiving a message that a change in at least one of bit rate andspatial resolution of a bit stream has occurred and setting the flagbased on the change of at least one of the bit rate and spatialresolution.
 11. The method of claim 2, wherein the flag comprises aHandleCraAsBlaFlag.
 12. The method of claim 2, wherein: the flagcomprises a first flag and a second flag; wherein the second flagcomprises a no_output_of_prior_pictures_flag; and wherein the secondflag also comprises a context variable.
 13. A video decoder forprocessing video data, comprising a processor configured to: receive anexternal indication at a video decoder; and treat a clean random access(CRA) picture as a broken link access (BLA) picture based on theexternal indication.
 14. The video decoder of claim 13, wherein theexternal indication indicates whether a flag should be set in the videodecoder, and wherein the processor is further configured to: set theflag based on the external indication; and check the flag, whereintreating the CRA picture as a BLA pictures based on the externalindication comprises treating the CRA picture as a BLA pictures based onthe flag.
 15. The video decoder of claim 14, wherein the flag isassociated with the CRA picture.
 16. The video decoder of claim 14,further configured to: receive an external indication that the flagshould be set; and set the flag based on the external indication. 17.The video decoder of claim 14, wherein a default value of the flag is“0” and a set value of the flag is “1.”
 18. The video decoder of claim14, further configured to, when decoding a coded slice NetworkAbstraction Layer (NAL) unit, if the flag is set, changing a NAL unittype of the NAL unit.
 19. The video decoder of claim 18, wherein theflag comprises a first flag and the video decoder is further configuredto, when decoding the coded slice Network Abstraction Layer (NAL) unit,if the flag is set, changing a value of a second flag.
 20. The videodecoder of claim 18, wherein the flag comprises a first flag, the videodecoder further comprising, when decoding the coded slice NetworkAbstraction Layer (NAL) unit, if the flag is set, setting the value of asecond flag to
 1. 21. The video decoder of claim 18, further configuredto decode a coded slice NAL unit includes parsing each coded slice NALunit to identify the NAL unit type.
 22. The video decoder of claim 21,wherein the flag comprises a first flag and when decoding a coded sliceNetwork Abstraction Layer (NAL) unit, if the first flag is equal to 1and a NAL unit type of a coded slice NAL unit indicates a CRA picture:changing the value of the NAL unit type to indicate a BLA picture. 23.The video decoder of claim 14, wherein setting the flag furthercomprises receiving a message that a change in at least one of bit rateand spatial resolution of a bit stream has occurred and setting the flagbased on the change of at least one of the bit rate and specialresolution.
 24. The video decoder of claim 14, wherein the flagcomprises a HandleCraAsBlaFlag.
 25. The video decoder of claim 14,wherein the flag comprises: a first flag and a second flag; wherein thesecond flag comprises a no_output_of_prior_pictures_flag; and whereinthe second flag comprises a context variable.
 26. A video decoder forprocessing video data, comprising: means for receiving an externalindication at a video decoder; and means for treating a clean randomaccess (CRA) picture as a broken link access (BLA) picture based on theexternal indication.
 27. The video decoder of claim 26, wherein theexternal indication indicates whether a flag should be set in the videodecoder, the video decoder further comprising: means for setting theflag based on the external indication; and means for checking the flag,wherein treating the CRA pictures as a BLA picture based on the externalindication comprises treating the CRA picture as a BLA pictures based onthe flag.
 28. The video decoder of claim 27, wherein the flag isassociated with the CRA pictures.
 29. The video decoder of claim 27,wherein a default value of the flag is “0” and a set value of the flagis “1.”
 30. The video decoder of claim 27, further comprising, whendecoding a coded slice Network Abstraction Layer (NAL) unit, if the flagis set, means for changing a NAL unit type of the NAL unit.
 31. Thevideo decoder of claim 30, the flag comprising a first flag and thevideo decoder further comprising, when decoding the coded slice NetworkAbstraction Layer (NAL) unit, if the first flag is set, means forchanging the value of a second flag.
 32. The video decoder of claim 30,the flag comprises a first flag, the video decoder further comprising,when decoding the coded slice Network Abstraction Layer (NAL) unit, ifthe flag is set, means for setting the value of a second flag to
 1. 33.The video decoder of claim 30, wherein decoding a coded slice NAL unitincludes parsing each coded slice NAL unit to identify the NAL unittype.
 34. The video decoder of claim 33, wherein the flag comprises afirst flag and when decoding a coded slice Network Abstraction Layer(NAL) unit, if the first flag is equal to 1 and a NAL unit type of acoded slice NAL unit indicates a CRA picture: changing the value of theNAL unit type to indicate a BLA picture.
 35. The video decoder of claim27, wherein setting the flag further comprises receiving a message thata change in bit rate of a bit stream has occurred and setting the flagbased on the change of the bit rate.
 36. The video decoder of claim 27,wherein the flag comprises a HandleCraAsBlaFlag.
 37. The video decoderof claim 27, wherein the flag comprises: a first flag and a second flag;the second flag comprises a no_output_of_prior_pictures_flag; and thesecond flag comprises a context variable.
 38. A computer-readablestorage medium having stored thereon instructions that, when executed,cause one or more processors of a device to: receive an externalindication at a video decoder; and treat a clean random access (CRA)picture as a broken link access (BLA) picture based on the externalindication.
 39. The computer-readable storage medium of claim 38,wherein the external indication indicates whether a flag should be setin the video decoder, the computer-readable storage medium wherein theinstructions, when executed, cause the one or more processors of adevice to: set the flag based on the external indication; and check theflag, wherein treating the CRA picture as a BLA pictures based on theexternal indication comprises treating the CRA pictures as a BLA picturebased on the flag.
 40. The computer-readable storage medium of claim 39,wherein the flag is associated with the CRA picture.
 41. Thecomputer-readable storage medium of claim 39, further configured tocause the one or more processors to: receive an external indication thatthe flag should be set; and set the flag based on the externalindication.
 42. The computer-readable storage medium of claim 39,further configured to cause the one or more processors to, when decodinga coded slice Network Abstraction Layer (NAL) unit, if the flag is set,change a NAL unit type of the NAL unit.
 43. The computer-readablestorage medium of claim 42, wherein the flag comprises a first flag andthe computer-readable medium is further configured to cause the one ormore processors to, when decoding the coded slice Network AbstractionLayer (NAL) unit, if the first flag is set, change the value of a secondflag.
 44. The computer-readable storage medium of claim 42, wherein theflag comprises a first flag and the computer-readable medium is furtherconfigured to cause the one or more processors to, when decoding thecoded slice Network Abstraction Layer (NAL) unit, if the flag is set,set the value of a second flag to
 1. 45. The computer-readable storagemedium of claim 42, further configured to cause the one or moreprocessors to decode a coded slice NAL unit includes parsing each codedslice NAL unit to identify the NAL unit type.